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Geology is the Way

Tourmaline

Trigonal

XY3Z6Si6O18(BO3)3(O,OH)3(O,OH,F)

The tourmaline group is a family of boron-bearing aluminium cyclosilicates that contain alkali elements (Na, Ca and even Li), Fe, and Mg. Notable members of the tourmaline group include elbaite, schorl, and dravite, which are common accessories in metamorphic and igneous rocks (especially in pegmatites) and are highly appreciated as gemstones, for their beauty and elevated hardness (7 on the Mohs scale). ‘Tourmali’ is a Sinhalese word which was locally used to describe colored gemstones from Sri Lanka (Ceylon), believed to be zircons during colonialism. When some of these ‘zircons’ arrived in the Netherlands, they were found to be a new mineral, which was ultimately named ‘tourmalin’ and ‘tourmaline’ by Rinmann (1766) and Richard Kirwan (1794).

Structure and chemistry
Tourmaline is characterized by the general formula XY3Z6Si6O18(BO3)3(O,OH)3(O,OH,F). Si-bearing tetrahedrons are organized in ditetrahedral rings with formula Si6O18 that lie on the basal plane of tourmaline (0001) and have their vertices pointing in the same direction and oriented parallel to the c-axis. The Y and Z octahedral sites are coordinated by O, OH, and F ions and are also organized in ditrigonal structures, in which the Y sites form an inner ‘ring’ of 3 edge-sharing octahedrons and the Z sites constitute an outer ring of 6 octahedrons that share an edge with Y octahedrons. The (O,OH, F) anion site is centered between the Y octahedrons, whereas the (O,OH)3 sites lie at the contact between two Z and one Y octahedrons. The Y site is larger than the Z site and contains Mg, Fe2+, Mn, Al, and Li, while the Z site contains mainly Al, Fe3+, and Mg. Finally, [BO3] triangular groups occur oriented perpendicular to c, sharing two corners with Z sites and one with the large cation site X, which contains Na, Ca, K or can be vacant. This structure, shown simplified in the image below, is piled up along the c-axis to have the X site on top of the tetrahedral ring and below the Y3Z6 octahedral sites and [BO3]  groups (see below).

Tourmaline crystal structure sketch

Simplified sketch of the crystal structure of tourmaline, as seen on the basal plane (0001) (left) and with a detail of the stacking of tetrahedral and cation sites along c (right). Modified after Deer et al., (1992).

Tourmalines can be divided in three first-order groups based on the occupancy of the X site (according to Hawthorne and Henry, 1999): alkali tourmalines (X = Na), calcium tourmalines (X = Ca), and X-vacant tourmalines (X = ). Subsequently, specific names can be assigned based on the content of Y and Z sites. There are many tourmaline minerals (full list on Mindat), but the most common species in rocks are dravite [NaMg3Al6Si6O18(BO3)3(OH)3(OH,F)], schorl [NaFe2+3Al6Si6O18(BO3)3(OH)3(OH,F)], elbaite [Na(Li1.5Al1.5)Al6Si6O18(BO3)3(OH)3(OH,F)] (all alkali tourmalines), uvite [CaMg3MgAl5Si6O18(BO3)3(OH)3(OH,F)], and liddicoatite [Ca(Li2Al)Al6Si6O18(BO3)3(OH)3(OH,F)] (belonging to the calcium tourmaline subgroup). All tourmalines can have oxy-, hydroxyl, or fluor-varieties, depending on the content of the anionic sites (e.g. hydroxilelbaite if the anion sites of an elbaite contain mainly OH). Natural tourmalines are characterized by many cationic and anionic substitutions (notably Fe2+ → Mg, Fe3+ → Al, Na K) which determine extensive solid-solutions. The solid-solution between alkali and calcium tourmalines is steered by the substitution of Na+ for Ca2+ in the X-site balanced by the substitution of Al3+ by Mg2+ in the Z site. Tourmalines also incorporate wide range of trace elements, like Mn, Ti, Cr, Ni, and V and, for this reason, they represent a ‘garbage can’ mineral from a geochemical perspective.

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Association of prismatic tourmaline crystals in a deformed pegmatite. Width: 3 mm. Calamita, Island of Elba, Italy.

Properties
Habit: prismatic
Hardness: 7
Density: 2.9 – 3.2 g/cm3
Cleavage: {11-20}, {10-11} very poor
Twinning: {10-11}, {40-41} rare
Color:
• dravite: black to brown
• schorl: generally black
• elbaite: blue, green, yellow, red, colorless
• uvite: brown, green, deep red
Luster: vitreous to resinous
Streak: brown to white
Alteration: –
In thin section…
ε: 1.612-1.650
ω: 1.633-1.671

Color:
• dravite: dark to pale yellow/brown
• schorl: green/blue to yellow
• elbaite: generally colorless
• uvite: pale yellow to colorless
Pleochroism: ω > ε
• dravite: ω dark brown/yellow brown, ε pale yellow/yellow
• schorl: ω dark green/blue, ε reddish violet/pale green/pale yellow
• elbaite: generally colorless and non pleochroic
• uvite: ω pale yellow, ε colorless
Birefringence (δ): 0.017-0.035 (second-order colors)
Relief: moderate
Optic sign:
[Mindat]

Field features

tourmaline crystal habit and optical properties

Some common crystal habits of tourmaline (left) and top view of a tourmaline crystal (right). Modified after van Hinsberg et al. (2011).

Tourmaline is most commonly found as visible crystals within granitic rocks, especially pegmatites, aplites, and associated hydrothermal veins. It is present also in metamorphic rocks as an accessory phase and in sedimentary rocks as a detrital grain but it is generally too small therein to be detected in the field. Tourmalines of the schorl-dravite series often appears black or brown and shows a prismatic habit, resembling pyroxenes and amphiboles at first glance. Despite this first-order similarity, tourmaline is much harder than most mafic minerals (hardness: 7, like quartz), and lacks evident cleavage planes. Moreover, euhedral tourmaline grains show the typical ditrigonal basal section (shown above). Other tourmaline species like elbaite are less common in rocks and mostly found in pegmatites. Elbaite is appreciated by mineral collectors for its transparency and wide range of colorations (pinks, reds, greens and blues). Another characteristic feature of tourmaline is the common presence of growth striations, elongated parallel to the long axis.

Tourmaline schorl

Prismatic crystals of tourmaline var. schorl. Photo © James St. John.

Tourmaline Elbaite from Elba

Tourmaline var. Elbaite from its type locality: San Piero in Campo, Island of Elba, Italy. Size: 3.36 cm. Photo © Didier Descouens.

oriented tourmaline

Oriented tourmaline grains (black) associated with quartz and alkali feldspar in a pegmatite dike. Capo Calvo, Calamita, Island of Elba, Italy.

tourmaline pegmatite

Prismatic grains of tourmaline (black) associated with partially transparent milky quartz and white/orange alkali feldspar. Naregno, Calamita, Island of Elba, Italy.

tourmaline metasomatic vein

‘Tourmalinization’. The phyllosilicate-rich layers of these schists were converted to black tourmaline after reacting with hydrothermal fluids in this vein. Quartz layers (white) were not affected by the reaction and survive in the core of the vein. Capo d’Arco, Island of Elba, Italy. More details: Dini et al. (2008).

Tourmaline in thin section
The characteristic pleochroism, prismatic habit, and lack of cleavage are generally diagnostic. The most common tourmaline species (dravite, schorl) display strong pleochroism at plane polarizers in shades of brown, yellow, and green, very similar to that of biotite and hornblende. However, contrarily to tourmaline, both biotite and hornblende show greater absorption when their long axis is parallel to the polarizer (more details below). Dravite shows brown to yellow pleochroic colors, whereas schorl is characterized by green to blue, yellow and reddish violet hues. Variations in colors and pleochroism are common in strongly colored tourmalines and generally highlight internal changes in composition (zoning patterns). Uvite is much paler, with very weak pleochroic colors (pale yellow to colorless), while pure elbaite is colorless and generally non pleochroic. Such, weakly pleochroic tourmalines can be confused with topaz, apatite, and corundum.
At crossed polars, tourmaline shows moderate second-order interference colors, which are usually masked by the intense colors and pleochroism.


Pleochroism of tourmaline (dravite-schorl). PPL. Width: 3 mm.


Concentric zoning in tourmaline. PPL. Width: 3 mm.

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Prismatic section of tourmaline, set in a matrix of recrystallized quartz. Width: 1 mm.

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Basal section of tourmaline (extinct at CPL), showing the typical trigonal section. Width: 3 mm.

Tourmaline vs biotite
As shown in the video below, tourmaline (the crystal in the center) and biotite (the platy grains around) show very similar color and pleochroism. The main difference is that tourmaline shows darker colors (maximum absorption) when its long axis is oriented vertical (perpendicular to the polarizer), whereas biotite shows maximum colors when its longer side is horizontal (parallel to the polarizer). To summarize:
Tourmaline: maximum colors N-S
Biotite: maximum colors E-W

Hornblende also shows maximum colors when its long axis is oriented E-W.

Gallery – Deformed tourmaline leucogranites
Deformed leucogranites (metaleucogranites) where tourmaline grains occur surrounded by recrystallized quartz, alkali feldspar (orthoclase), and plagioclase. Sericite is present as an alteration product of feldspar. Some samples also contain white mica (muscovite). For more details: Papeschi et al. (2022).
Sample courtesy Giovanni Musumeci.

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Concentric zoning in tourmaline. Width: 3 mm.

Gallery – Deformed tourmaline pegmatite
Quartz + tourmaline + plagioclase pegmatite deformed in a shear zone within schistose rocks of the contact aureole of the Calamita (mineralogy of the host rocks: biotite + quartz + andalusite + cordierite + muscovite). For more details: Papeschi et al. (2022).
Sample courtesy Giovanni Musumeci.

tourmaline pegmatite

Bent and deformed tourmaline grains, showing undulose extinction. CPL. Width: 3 mm.

faulted tourmaline

Microfaulted tourmaline grains surrounded by recrystallized quartz. CPL. Width: 3 mm.

healed fractured tourmaline

Fracture in brown tourmaline (dravite) filled by blue tourmaline (schorl). PPL. Width: 3 mm.

Occurrence
Tourmaline is a typical mineral of evolved granitic rocks, especially pegmatites but also leucogranites and associated veins. The association with granitic intrusions is related to the fact that incompatible elements, like boron and lithium, become concentrated in the residual granitic melt, allowing the crystallization of tourmaline in late-stage igneous and hydrothermal rocks. Around plutons, hydrothermalism can also led to crystallization of tourmaline as a metasomatic product in aureole or country rocks . This process, called ‘tourmalinization’, is marked by the replacement of ferromagnesian minerals, like biotite, by tourmaline.
Metamorphic rocks, especially metasediments, often contain dravite-schorl tourmaline as an accessory. In these rocks, tourmaline forms as a recrystallization of detrital tourmaline present in the sedimentary protolith or due to boron metasomatism. Argillaceous sediments may also contain some boron, absorbed by clays, which recrystallizes as tourmaline during metamorphism. Magnesian tourmalines may also form during metasomatism of basic rocks. Tourmaline is also present in marbles, calcschists and calcsilicate rocks, likely with uvite composition.

References
Hawthorne, F. C., & Henry, D. J. (1999). Classification of the minerals of the tourmaline group. European Journal of Mineralogy 11, 201-215.

        

The information displayed on this page is after Introduction to the Rock-Forming Minerals and Optical Mineralogy: Principles and Practice.

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